Pulse-width modulation (PWM) is a technique for encoding a message into a pulsing signal. Although this modulation technique can thus be used to encode information for transmission, its main use is to allow control of the power supplied to electrical devices, especially to inertial loads such as motors.
The average value of voltage (and current) fed to a load is controlled by turning a switch between supply and load on and off at a fast rate. The longer the switch is on compared to the off periods, the higher the total power supplied to the load.
The PWM switching frequency has to be much higher than what would affect the load (the device that uses the power), such that the resultant waveform perceived by the load must be as smooth as possible. Typically switching has to be done several times a minute in an electric stove, 120 Hz in a lamp dimmer, from few kilohertz (kHz) to tens of kHz for a motor drive and well into the tens or hundreds of kHz in audio amplifiers and computer power supplies.
The term duty cycle describes the proportion of “on” time to the regular interval or period of time; a low duty cycle corresponds to low power, because the power is off for most of the time. Duty cycle is expressed in percent, 100% being fully on.
The main advantage of PWM is that power loss in the switching devices is very low. When a switch is off, there is practically no current, and when it is on and power is being transferred to the load, there is almost no voltage drop across the switch. Power loss, being the product of voltage and current, is thus in both cases close to zero. PWM also works well with digital controls, which, because of their on/off nature, can easily set the needed duty cycle.
PWM can be used to control the amount of power delivered to a load without incurring the losses that would result from linear power delivery by resistive means. Drawbacks to this technique are that the power drawn by the load is not constant but rather discontinuous, and energy delivered to the load is not continuous either. Power flow from the supply is not constant and will require energy storage on the supply side in most cases.
PWM power control systems are easily realisable with semiconductor switches such as MOSFETs or insulated-gate bipolar transistors (IGBTs). As explained above, almost no power is dissipated by the switch in either on or off state. However, during the transitions between on and off states, both voltage and current are nonzero and thus power is dissipated in the switches. By quickly changing the state between fully on and fully off, the power dissipation in the switches can be quite low compared to the power being delivered to the load. The use of synchronous switch topologies such as a half-bridges or synchronous buck converters, further reduces power losses, but leads to a significant increase of circuit complexity.
In a half-bridge, synchronous buck converter, or other synchronous switch topologies, a switch driver has to be implemented to prevent both switches from being turned on at the same time, a fault known as “shootthrough”. The simplest technique for avoiding shootthrough is a time delay between the turn-off of a first switch to the turn-on of second switch, and vice versa. However, setting this time delay long enough to ensure that the first and second switch are never both on at the same time will itself result in excess power loss.
Thus, in switched mode power supplies, when using synchronous switch topologies, such as half-bridges or synchronous buck converters, insertion of a dead time between the turn on and turn off of the complementary switches is required. While this dead time insertion avoids shorts between the switches, it will impact the overall efficiency of the converter itself if the length of the dead time is not correctly set. The required dead time is however not constant as it not only depends on the operating conditions of the converter, such as temperature variation or degradation, but also depends on the current conditions of the system as the optimal value also depends on the output load of the power supply. This imposes that the dead time has to be modified/updated on-the-fly via a control loop.
In current or at least future synchronous topologies, the dead time, due to the increasing switching frequency of the converters, needs to be adjusted with very fine granularity (sub nanosecond range) together with the ON/OFF of the PWM duty cycle. But this imposes that a counter needs to be started upon a transition in the PWM signal (ON or OFF) that contains the actual value of the dead time, e.g. 5.5 ns and be decoded on-the-fly. This means that it is currently very complicated, if not impossible to decode a counter value on-the-fly in the order of picoseconds and apply a gating/ungating on the PWM signal to generate this highly accurate dead time.
For these or other reasons there is a need for an improved method and/or apparatus for controlling current in an array cell.